In the production of today's integrated circuits, optical lithography is a key processing step. The ongoing miniaturization of integrated circuits or other devices produces a number of problems, which may be encountered during optical lithography.
In an optical lithographic system, when light is incident on a mask, the light may be diffracted. The smaller the dimensions of the structures on this mask, the more the light will spread. Hence, the smaller the dimensions of the structures on the mask, less of this spread-out light will be collected by an objective lens so as to be focused onto a resist layer. As a result, the image of the mask structure formed onto the resist layer may be of a low quality. The collection of light can be improved by choosing a lens with a high numerical aperture (NA). The use of such high NA lenses results in a high angle of incidence of the focused light onto the wafer and in a reduction of the depth of focus. The problem of high angle of incidence can be overcome by introducing a medium with a higher refractive index, i.e. higher than air, in-between the lens and the resist, which is typically done in case of immersion lithography.
The quality of the printed image in optical lithography depends on the polarization state of the light used, which has an influence on the transmission, reflection and interference properties in a resist layer. As the polarization state of the light typically changes during its propagation through the optical system, it is difficult to ascertain the affect of using light initially having a given polarization state. The presence of large angles of diffraction and large angles of incidence, due respectively to the shrinking dimensions of mask structures and the use of high NA lenses in the optical system, will make light propagation even more prone to the polarization status of the transmitted and/or reflected light, resulting in a non-obvious, non-straightforward correlation between the polarization behaviour of the light source and the obtained quality of a printed image. It is to be noted that the polarization behaviour of the light source may be uniform or non-uniform, meaning that the polarization state of the light source may be identical or varying between different points in the light source.
In order to obtain a correct image or print of the mask info onto the resist layer upon the wafer and/or in order to be able to estimate the influences of polarization and diffraction on the image or print of the mask info onto the resist layer upon the wafer, it would be useful to obtain information about the amount of light and the polarization state of the light reaching a resist layer in a lithographic system. Some efforts have already been made in order to take into account the influence of the polarization state of light used in optical lithographic processing.
For example, methods are known for obtaining useful information about the light transmission and its polarization state. Typically, Maxwell equations are used to describe the optical effects in an optical system, as described in more detail in “Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (7th edition)” by Born and Wolf, Cambridge United Press 1999. The use of Maxwell equations to describe the complete optical system results in very tedious and labour-intensive methods or methods that do not yield a solution. Alternatively, in order to overcome complexity, current methods often make use of a reduced set of equations, corresponding to a simplified description of the optical system, resulting in errors and incorrect understanding, and leading to less optimal optical lithographic processing.
US patent application US-2004/0119954 A1 describes a method and apparatus for preventing deterioration of the imaging performance due to the influence of polarization. The document describes a method and apparatus such that, for light having a specific angle of incidence, the light only includes s-polarized light. The number of existing optical lithographic systems fulfilling these requirements is limited. Typically, a polarization control part is used. Nevertheless, as the polarization at the device level typically will not be uniform, even if a—theoretically—perfectly polarized source and a non-polarized mask and lens are used, a mixture of s-polarized light and p-polarized light will always be present, and therefore a p-polarized light component will need to be dealt with anyway.
None of the above cited methods or systems allows to relatively easily obtain useful information related to the quality of the processing obtainable for optical lithographic systems, without compromising too much of the existing optical lithographic systems.